1. Field of the Invention
The present invention relates to a method of and an apparatus for forming multistage hollow pipe, namely, for forming multistage bulges and spools on the hollow pipe.
2. Prior Art
The simplest method to buckle a hollow pipe is by fixing the hollow pipe by a plurality of molds and applying a compressive force to the hollow pipe in a punching press or a drawing press. In this method, there is required an apparatus for returning the plurality of molds to their own positions for carrying out the next step at the time of buckling or pressing the hollow pipe, namely, at the time when the molds are moved simultaneously with the buckling of the hollow pipe.
However, if the molds are to be returned immediately after the completion of the buckling of the hollow pipe, and if at least three bulges or spools are formed on the hollow pipe, the molds can not return to their own positions because the bulges or spools formed on the hollow pipe impede the return of the molds. If the molds are forced to be returned, the molds damage the bulges or spools. There arises the problem set forth above even if a spring is employed, although it has been employed in most cases, for returning the molds since it is liable to return immediately after releasing the compressive force applied to the hollow pipe.
In view of the problems set forth above, a plurality of steps are conventionally taken for forming at least three bulges or spools on the hollow pipe without taking these steps simultaneously.